A filling-level measuring device that evaluates echo signals
The present invention relates to a filling-level measuring device which emits a signal for the purpose of determining the filling level of a material in a container and which evaluates a received echo of the signal.
So-called filling-level radar systems which operate according the method indicated above are known to the prior art. Current filling-lever radar systems emit a pulsing radio signal with an established center frequency fM and a bandwidth B. The transmitting and receiving antenna of these filling-level measuring radar systems are adjusted to this frequency range.
These filling-level measuring radar systems are used to monitor the filling level of a number of substances in containers of differing materials. The received echo signal is composed of contributions from the monitored filling level, the walls of the container, and other possible sources of interference; here the ratio of the intensities of the individual echo contributions can vary strongly, depending on the substance being monitored, the material of the container wall, and the working frequency of the radar. Thus the measuring accuracy of this kind of filling-level measuring radar may vary considerably depending on the application environment.
This state of affairs is not desirable, either for the users of these filling-level measuring radar systems, who are interested in reliable measurement, or for the manufacturers of such systems, who receive requests for specifications on the measuring accuracy of the system and who run the risk of being held liable if they give too optimistic an appraisal of the measuring accuracy and of seeing the customer choose a different system if the appraisal is too conservative. Thus there is considerable need for a filling-level measuring device with a transmitter, a receiver for an echo emitted by the transmitter, and an evaluating circuit for estimating the filling level on the basis of the transit times of different contributions of the echoxe2x80x94a device which can deliver measuring results of essentially unchanging accuracy for a number of application environments, more precisely, for a number of combinations of monitored substances, container materials, etc.
The invention solves this problem with a filling-level measuring device of the indicated type in which the transmitter and the receiver are suitable for operation with a plurality of frequencies on the part of the transmitted signal and the echo. When installed, e.g., in the application environment of a testing phase, this kind of filling-level measuring device permits the testing of various available frequencies andxe2x80x94when measurements are performed in long-term operationxe2x80x94allows selection of the frequency that provides the best measuring accuracy, e.g., the best signal-to-interference ratio or the most intensive echo from the surface of the substance being monitored.
This kind of filling-level measuring device may comprise a transmitter and receiver module which can be adjusted to the same frequency, as ""selected from a set of frequencies.
In a preferred embodiment the frequency of the transmitter and receiver module can also be changed during operation of the device. In this way it is possible to receive echoes on all frequencies of the set and, for example, to employ the best echo in estimating the filling level in the evaluation circuit.
It is expedient for the change in frequency to be performed in cyclical fashion by the filling-level measuring device itself.
In a second preferred embodiment the transmitter is suitable for transmitting simultaneously on a number of frequencies and the receiver comprises a number of receiver modules which are suitable for receiving a number of frequencies. With this kind of filling-level measuring device a plurality of echo signals can be obtained at the different frequencies in relation to the same measuring point, and the echo signals are emitted at the same time from the receiver modules, which facilitates processing in the evaluating circuit as compared to a successive generation of the echo signals.
Another improvement in measuring accuracy can be achieved by linking the echo signals obtained at the different frequencies. To filter out the interference contributions it is expedient to assure that the evaluating circuit takes into account only those contributions that occur in at least two echo signals of different frequency, or even more strictly, in all echo signals.